What I have found to be the best approach to dialing in a certain sound is to find what the artist was using for equipment and then devloping a patch using the same Line 6 emulations.  If not possible going through all of the patches and then finding one that is closest to what I am looking for and then start tweaking the settings to see what effect they have.  There really isn't any way to explain how to do this that is better than you yourself experimenting.  Also going to the custom tone section and downloading specfific artist patches can be helpful.  Just a warning though some of the patches listed can sound totally different then advertised.  This primarily due to the guitar/amp combination that was used to develop the tone.  I.e. a Stratocaster going into a Mesa Boogie.